This invention relates generally to digital signal space division switching systems and more particularly to an electronically controlled cross-connect system using a space division switching network to cross-connect digital signals may be synchronous or asynchronous to one another.
In a geographically dispersed transmission network consisting of "nodes" interconnected by node-to-node transmission facilities, a typical node will include a number of transmission terminals which send and receive signals from transmission terminals at other nodes, and an intra-node cross-connect system which routes signals between the transmission terminals at that node. A rudimentary cross-connect system is simply an array of jacks providing signal access to transmission terminals and patch cords providing jack-to-jack connections. The rudimentary cross-connect system can be reconfigured by manually rearranging the patch cords. However, the rudimentary cross-connect system is functionally and administratively inadequate for large modern networks with high traffic volume and frequent requirements for circuit re-routing. Such networks require large cross-connect systems which can be reconfigured electronically from a local control terminal or remotely from a network control master station. An electronic cross-connect system is a switch network which provides electronically switchable paths between its inputs and outputs.
For administrative reasons, the signals connected to the cross-connect system are all of the same type. In particular, for a digital cross-connect system the signals are all digital and all at the same nominal bit-rate, although not necessarily synchronous with one another.
Digital signals can be switched by time division methods, or space division methods, or a combination of both. Time division methods use multiplexing of low-bit-rate space-divided signals into higher-bit-rate time-divided signals, followed by time slot interchanges, followed by demultiplexing to lower-bit-rate space-divided signals. If the space divided signals to be switched are already at high bit-rates, it may be impractical to multiplex them to higher rates for switching purposes, but high-bit-rate signals can be switched by space division methods. This invention relates to space division switching networks for high-bit-rate digital signals, but the principles of the invention are also applicable to low-bit-rate digital signals.
The theory of space division switch networks was developed in terms of electro-mechanical switching machines for low frequency analog signals, wherein metallic crosspoints make or break metallic paths through the network. Such networks are inherently capable of transmission in either direction without signal conditioning, even in very large networks. Such networks are inappropriate for high-bit-rate digital signals.
High-bit-rate digital signals are more appropriately switched in networks of "large scale integrated" semiconductor devices wherein logic gates provide the crosspoint function. Such networks are inherently capable of transmission in one direction only, and the digital signals may require regeneration (i.e., reshaping and retiming) as they traverse the network.
A well-known class of high capacity space division switching network is the multistage co-ordinate switching network described by C. Clos in "A Study of Non-Blocking Switching Networks," Bell System Technical Journal, March 1953. A Clos network is non-blocking in the sense that a new path can always be connected from any input to any output without changing any of the already-connected internal paths in the network. Alternatively, a non-blocking "rearrangeable" network is one in which a new path can always be connected from any input to any output, provided one is allowed to rearrange already-connected internal paths without changing their endpoints. The advantage of a rearrangeable network over a Clos network is a major reduction in total number of crosspoints. The disadvantage is that, when a new connection is made, the rearrangement of internal paths may disturb existing through traffic. An object of the present invention is to provide a rearrangeable switching network for high-bit-rate digital signals in which existing through traffic is not disturbed when a new connection is made.
A high-bit-rate digital signal propagating through a large space division switch may be seriously degraded as it passes through a succession of crosspoints (logic gates) along its path. For reliable transmission through the switch network, every through signal may require regeneration (reshaping and retiming) at one or more stages within the switch network. Digital signal regeneration requires a clock waveform for retiming. If the through signals are all synchronous and bit-aligned to a common clock, then this one clock can be used for all regenerators within the switch network. However, if the through signals are asynchronous, then each one requires its own clock for regeneration. Another object of the present invention is to provide a space division switch network with internal signal regeneration for high-bit-rate digital signals which requires only one clock frequency for internal regenerators regardless of whether the through signals are synchronous or asynchronous. This object is achieved by converting asynchronous digital through signals to bit streams which are synchronous within the switch network.